In the C code (app/test-pmd/parameters.c) "loop" is used instead of "loopback".
The run_app.rst file should use "loop" as well.
> In ``paired`` mode, the forwarding is between pairs of ports, for
> example:
> (0,1), (2,3), (4,5).
>
> @@ -204,6 +204,8 @@ The commandline options are:
>
> The ordering of the ports can be changed using the portlist testpmd
> runtime
> function.
>
> + In ``loopback`` mode, ingress traffic is simply transmitted back on the
> same
> interface.
> +
> * ``--forward-mode=mode``
>
> Set the forwarding mode where ``mode`` is one of the following::
> --
> 1.8.3.1
Regards,
Bernard.